556 T. DAVIES ON THE MICROSCOPIC STRUCTURE 
minute enclosures other than fluid, causing its well-known dirty 
aspect, is abundant. The felspars of both rocks are of the same 
character and habit, although necessarily more fragmentary in the 
conglomerate. Though not abundant they are there, and can be 
mostly distinctly recognized. In some cases they are not more 
altered than in the Dimetian; but in others the structure has 
entirely disappeared, leaving a kaolin-like mass which feebly 
depolarizes light. 
The contents of the Cambrian conglomerates of St. David’s found 
in the sections examined are :— 
Quartz, both dirty and clear. 
Small pebbles consisting of dirty quartz with two felspars. 
Individual felspars, both orthoclase and plagioclase. 
Felsites. 
Quartz-felsite. 
Quartz-schists. 
Quartzite. 
Basic rocks. 
Porcellanite. 
Mica much altered. 
These are all cemented by a magma which appears to be more or 
less of felspathic origin. 
No. 72, from Treglemais, is a hialleflinta-like rock, apparently 
brecciated in situ. It is of exceedingly minute grain and encloses 
a few fragments of felspar and of quartz. 
No. 73. Roch Castle. This section has already been described 
(Quart. Journ. Geol. Soc. vol. xxxv. (1879), p. 291, No. 2). I have 
but to reiterate that this rock contains undoubted sharp angular 
fragments of quartz. 
No. 74. Roch Castle. A similar rock to No. 73, brecciated. 
No. 75. Roch Castle. Macroscopically this is a dull siliceous 
rock containing small fragments of a dull white, and larger 
fragments of a bluish-grey hornstone-like rock. In thin section it 
presents a similar structure to Nos. 73 and 74, but contains angular 
pieces of a dense rock which has no action on polarized light except 
in minute points. 
No. 76. Roch Castle. A similar rock, but of a reddish colour; 
the included fragments are not so large. In structure it is similar to 
the preceding, but somewhat more coarsely crystalline; it is much 
obscured by abundance of iron oxide. ) 
No. 77. Roch Castle. A hornstone-like fragment similar to that 
enclosed in No. 75, but of a coarser texture and more quartzose in 
aspect, with some attached fragments of a yellowish compact rock. 
The section shows an exceedingly minutely crystalline ground- 
mass almost inert between crossed nicols, with much quartz in 
irregular grains distributed in patches and groups. The yellowish 
